1. Field of the Invention
The present invention relates to gyroscopes and input devices using the gyroscopes. More specifically, the present invention relates to a gyroscope in which displacements of the tines of a tuning fork, which occur when an angular velocity is applied, are detected by using variations in capacitances, and to an input device using the gyroscope.
2. Description of the Related Art
Conventionally, gyroscopes in which a tuning fork formed of a conductive material such as silicon, etc., is used are known. In these types of gyroscopes, the tines of the tuning fork are vibrated in one direction, and vibrations thereof in the direction perpendicular to this direction, which occur due to Coriolis force when an angular velocity about the central axis parallel to the longitudinal direction of the tines is input, are detected. The vibrations which occur due to Coriolis force correspond to the angular velocity applied. Thus, the gyroscopes may be used as angular velocity sensors, and may be used in, for example, coordinate input devices for personal computers.
FIG. 15 is a schematic diagram showing a construction of a conventional gyroscope, which is disclosed in Japanese Unexamined Patent Application Publication No. 11-311520 which is assigned to the present assignee. As shown in FIG. 15, a gyroscope 100 includes a tuning fork 103 having three tines 101 and a supporting portion 102 which connects base ends of the tines 101. The tuning fork 103 is formed of silicon which has electric conductivity. The supporting portion 102 is fixed on a substrate 104 formed of a glass, and drive electrodes 105a, 105b, 105c, and 105d, which are also formed of silicon, are disposed between and the tines 101 and outside the tines 101 at both ends. The drive electrodes 105a and 105c are electrically connected with each other, and the drive electrodes 105b and 105d are also electrically connected with each other. An alternating voltage having opposite phases is applied to the pair of drive electrodes 105a and 105c and to the pair of the drive electrodes 105b and 105d. Accordingly, electrostatic attractions occur when the voltage is applied to the drive electrodes 105a to 105d, and each of the tines 101 is vibrated in a direction parallel to the surface of the substrate 104. This direction will be referred to as the lateral direction in the descriptions hereof.
In the gyroscope 100, when an angular velocity about an axis parallel to the longitudinal direction of the tines 101 is input while the tines 101 vibrate in the lateral direction, vibrations of the tines 101 in the direction perpendicular to the substrate 104 occur. This direction will be referred to as the thickness direction in the descriptions hereof. The vibrations of the tines 101 in the thickness direction are detected by detection electrodes 106, which are disposed under the tines 101. The detection electrodes 106 are formed on the substrate 104 as metal films of chromium, etc. When the tines 101 vibrate in the thickness direction, the gaps between the tines 101 and the detection electrodes 106 vary, so that electrostatic capacitances between the tines 101 and the detection electrodes 106 also vary. Therefore, by obtaining the variations of electrostatic capacitances in terms of electric signals, the input angular velocity may be determined.
Generally, there are two types of such gyroscopes. In one type, which is referred to as a lateral direction driving type, the tines are driven in the lateral direction, and vibrations thereof in the thickness direction are used for the detection. In the other type, which is referred to as a thickness direction driving type, the tines are driven in the thickness direction and the vibrations thereof in the lateral direction are used for the detection. The gyroscope 100 shown in FIG. 15 is of the former type.
In the gyroscopes having the above-described construction, the drive electrodes are disposed at both sides of each of the tines. Thus, gaps between the tines cannot be made sufficiently small. More specifically, when the width of the drive electrodes is x1, and the gap between the drive electrodes and the tines is x2, the gap G between the tines is calculated as G=x1+2x2. There are limits determined by silicon processes using typical technologies for manufacturing semiconductor devices regarding the amounts by which x1 and x2 can be reduced. Accordingly, there is also a limit to how much the gap G between the tines can be reduced.
On the other hand, it is known that in three-tine type tuning forks, a xe2x80x9cQ valuexe2x80x9d, which indicates a degree of resonance in devices such as tuning forks, may be increased by reducing the gap G between the tines. When the Q value is increased, efficiency at which electric energy input to the device is converted into vibration energy is improved. Thus, in the lateral direction driving type gyroscope, a large driving force can be obtained using a small driving voltage. Therefore, the driving voltage can be reduced.
As described above, it is expected that various advantages can be obtained by reducing the gap between the tines; for example, the size of the device and the driving voltage can be reduced. In the conventional gyroscope, however, there is a limit to how much the gap between the tines can be reduced, and it has not been possible to achieve a reduction of the gap.
Accordingly, it is an object of the present invention to provide a low-cost, lateral direction driving type gyroscope in which the above-described various effect can be obtained, and an input unit using the gyroscope.
In order to attain the above-described object, according to an aspect of the present invention, a gyroscope includes a tuning fork having vibrating beams; a pair of substrates which are disposed one at each side of the tuning fork, at least the surfaces thereof being insulative; drive electrodes which are provided on each of the substrates in such a manner that parts of the drive electrodes oppose the vibrating beams and the remaining parts of the drive electrodes protrude from the vibrating beams, the drive electrodes being capacitively coupled to the beams and driving the vibrating beams in a direction parallel to the substrates; and detection electrodes which are capacitively coupled to the vibrating beams, and which detect displacements of the vibrating beams in a direction perpendicular to the vibrating direction of the vibrating beams.
The gyroscope of the present invention is assumed to be the lateral direction driving type. In addition, similar to the conventional type, the principle for driving the vibrating beams is based on an electrostatic attraction force. In the conventional gyroscope, vibrating beams (which corresponds to the above-described tines) of the tuning fork are driven by using attraction forces applied to the opposing surfaces of the vibrating beams and the drive electrodes. In contrast, in the gyroscope according to the present invention, the drive electrodes are disposed in such a manner that the parts thereof oppose the vibrating beams of the tuning fork and the remaining parts thereof protrude from the vibrating beams. Thus, when a voltage is applied between the vibrating beams and the drive electrodes, the vibrating beams are driven by forces applied in directions in which the opposing areas between the vibrating beams and the drive electrodes are increased.
In order to describe this more specifically, with reference to FIG. 11, a case is considered in which a vibrating beam and a drive electrode have surfaces which are shifted relative to each other in the horizontal direction (in FIG. 11) and which include opposing parts 1 and 2. When the size of the surfaces in the direction perpendicular to the shifting direction thereof is g and the distance between the surfaces is d, the electrostatic attraction force F applied in a direction in which the area of the opposing parts 1 and 2 is increased can be calculated as the following.
F=(1/2)xc2x7xcex50(g/d)V2xe2x80x83xe2x80x83(1) 
wherein xcex50 is the dielectric constant in vacuum, and V is an applied voltage.
Due to the force F calculated by equation (1), the vibrating beams vibrate in the direction parallel to the substrates on which the drive electrodes are provided (in the lateral direction). In the construction according to the present invention, the above-described size g can be increased in the longitudinal direction of the vibrating beams, so that a large driving force can be obtained. Conversely, the driving voltage for obtaining a predetermined driving force can be reduced. In the above-described construction, the drive electrodes are provided on each of the substrates disposed at both sides of the vibrating beams. When a voltage is applied between the vibrating beams and the drive electrodes, the vibrating beams receive not only the forces in the direction parallel to the substrates but also the forces in the direction perpendicular to the substrates. More specifically, with respect to FIG. 11, a force in the direction perpendicular to the opposing parts 1 and 2 (attraction force) will also occur in addition to a force in the direction parallel to the opposing parts 1 and 2 (in a direction in which the opposing area is increased). Thus, if the drive electrodes are provided at only one side of the vibrating cantilevers, the vibrating cantilever also vibrates in the direction perpendicular to the substrates (in the thickness direction).
The principle for detection is the same as that in the conventional type, in which the vibrations of the vibrating beams of the tuning fork are detected by variations of the electrostatic capacitances. More specifically, the variations of electrostatic capacitances, which occur when the vibrating beams are vibrated in the thickness direction due to Coriolis force and distances between the vibrating beams and the detection electrodes vary, are detected. The Q value of the detected vibration (vibration in the thickness direction) may be controlled by controlling the distances between the vibrating beams and the detection electrodes. Accordingly, the Q value of the generated vibration may be sufficiently increased, and the Q value of the detected vibration may be reduced to an adequate value. As a result, by increasing the Q value of the generated vibration and by reducing the Q value of the detected vibration, a broad detection characteristic may be obtained. Thus, a device in which vibrations of large degree are generated and which has stable detection sensitivity may be realized.
According to the gyroscope constructed as described above, when an angular velocity about an axis parallel to the longitudinal direction of the vibrating beams is input while they vibrate in the lateral direction, vibrations thereof in the thickness direction also occur due to Coriolis force. Since the vibrating beams and the detection electrodes are capacitively coupled, the electrostatic capacitances vary with the variations in the distances between the vibrating beams and the detection electrodes. Accordingly, the input angular velocity may be determined by detecting the variation of capacitances.
As described above, in the gyroscope having the above-described construction in which the vibrating beams are sandwiched and supported by the substrates from both sides thereof, the drive electrodes may be provided on the substrates in such a manner that the parts thereof oppose the vibrating beams. Thus, it is not necessary to dispose the drive electrodes between the tines and outside the tines as in the conventional type. Therefore, the gap between the tines may be reduced to, for example, a limit determined by silicon processes, and the Q value may be sufficiently increased. As a result, the driving voltage may be reduced, and, of course, the size of the device may be reduced.
Each of the substrates may be provided with a plurality of drive electrodes. In such a case, the drive electrodes are preferably disposed at both sides of central lines of the vibrating beams which are parallel to the longitudinal direction thereof.
By disposing the drive electrodes at both sides of the central lines of the vibrating beams which are parallel the longitudinal direction thereof and by alternately applying voltage to the drive electrodes, a vibration mode which is more stable may be easily realized. In addition, in a case in which the vibrating beams are vibrated across the central lines thereof, the drive electrodes are preferably disposed at symmetrically about the central lines so that the amplitudes of both sides of the central line become the same. However, the drive electrodes are not necessarily disposed symmetrically, as long as vibrations symmetrical across the central lines are realized.
In addition, the detection electrodes are provided on at least one of the pair of substrates. The vibrations of the vibrating beams in the direction perpendicular to the substrates may be detected in terms of variations of electrostatic capacitances between the vibrating beams and the detection electrodes. In addition, when the detection electrodes are provided on both of the substrates, the effect of noise may be reduced by performing the detection from both sides.
In addition, according to another aspect of the present invention, a gyroscope includes a tuning fork having vibrating beams; at least one substrate which is disposed at at least one side of the tuning fork, and which is insulative at at least the surface thereof; drive electrodes which are disposed in such a manner that parts of the drive electrodes oppose the end surfaces of the vibrating beams in the longitudinal direction thereof and the remaining parts of the drive electrodes protrude from the end surfaces of the vibrating beams, the drive electrodes being capacitively coupled to the vibrating beams and driving the vibrating beams in a direction parallel to the substrate; and detection electrodes which are capacitively coupled to the vibrating beams, and which detect displacements of the vibrating beams in a direction perpendicular to the vibrating direction of the vibrating beams.
In this gyroscope, instead of providing the drive electrodes on the substrates disposed at both sides of the vibrating beam, the drive electrodes are disposed in such a manner that parts of the drive electrodes oppose the end surfaces of the vibrating beams in the longitudinal direction thereof. Also in such a construction, when a voltage is applied between vibrating beams and the drive electrodes, the vibrating beams are vibrated in the lateral direction by electrostatic attraction forces applied in directions in which the opposing areas between the vibrating beams and the drive electrodes are increased. In addition, when the drive electrodes are disposed as described above, the vibrations of the vibrating beams in the direction perpendicular to the substrate does not occur while the vibrating beams are driven. Thus, the tuning fork is not necessarily provided with the substrates at both sides thereof as long as it is provided with the substrate at at least one side thereof.
Also in this gyroscope, each of the substrates may be provided with a plurality of drive electrodes. In such a case, the drive electrodes are preferably disposed at both sides of central lines of the vibrating beams which are parallel to the longitudinal direction thereof. In addition, the detection electrodes may be provided on the substrate.
In addition, according to another aspect of the present invention, an input device includes the gyroscope according to either one of the above-described aspects of the present invention. By using the gyroscope according to either one of the above-described aspects of the present invention, small devices such as coordinate input devices for personal computers may be realized.